This invention relates to actuating devices for plug or globe type valves and more particularly to such valves having a dual actuating device.
It is well known that in the introduction of a pressure loss by a valve within a system, the mode of actuation of said valve is an important design criterion. Heretofore, a variety of valve actuating devices were know including pneumatically operated (diaphragm) actuators, piston (cylinder) actuators, electro-hydraulic actuators, high-performance servo actuators, electro-mechanical actuators, and mechanical screw actuators.
As a type, mechanical screw actuators are the most familiar valve actuator. By rotation, mechanical screw actuators use the threaded mechanical advantage of the screw threads between the valve plug stem and a threaded mounting member of the valve body or between a threaded handle and the threaded valve plug stem to create an axial displacement of the valve plug.
The design of a mechanical screw actuator for a valve depends upon a variety of criteria. The most important of these criteria are the amount of pressure control within the system that is needed, the ability to sacrifice that pressure control for the convenience of actuation, the nature of the fluid to be controlled and the extent to which complete shutoff is required.
The amount of pressure control that can be exerted by mechanical screw actuators depends upon the number of threads per inch of the threaded valve stem. By use of a fine thread (many threads per inch) precise pressure control can be obtained since because of the large number of threads per inch, rotation of the valve stem can be effected to result in a small axial displacement of the valve plug. In contrast to this use of a fine thread for the threads of the valve stem is the use of a coarse thread (fewer threads per inch). By rotation of the valve stem with a coarse thread, only gross pressure control can be obtained, since because of the fewer number of threads per inch rotation of the valve stem results in a larger axial displacement of the valve plug.
Besides this limitation as to the amount of pressure control that can be effected from the aforementioned choice of threads is the fact that said choice of threads also effects the convenience by which said valve can be operated. The choice of a fine thread requires that the actuator be rotated many times for any substantial axial displacement of the valve plug stem while the choice of a coarse thread only requires a few rotations of the actuator for the same degree of axial displacement.
A further consequence of this choice as to a coarse or a fine thread in the design of the valve actuator is the ability to tightly seat the valve plug. The extent to which valve shufoff or complete pressure loss within a system can be effectuated depends upon the extent to which the valve plug can be tightly seated onto the valve seat port. Shufoff force is a direct function of the choice of the thread size for the valve stem threads. A fine thread size provides the greatest mechanical advantages and would allow a large seating force except that, by its design, the fine thread only allows for a relatively small force to be exerted to seat the valve plug. While a coarse thread size, because of its thread design, allows for a larger seating force to be exerted, but lacks the mechanical advantages of the fine thread.
In selection of a valve actuator, concern for the nature of the fluid that is to be controlled exists because of the intensive contact relationship between the valve plug and the valve seat. With mechanical screw actuators the valve plug is either rotated until tightly seated onto the valve seat or as in handwheel type of valves the valve plug is axially seated onto the valve seat. The effect of this contact rotation or axial seating is the progressive uneven wear of the valve seat or the valve plug or both. In addition, in those applications where the fluid in the system has an abrasive component the wear of the valve seat or plug due to the rotational or axial grinding of the valve plug on the valve seat is even more pronounced.